1. Field of the Invention
The present invention relates to vertical unipolar components and more specifically to the periphery of such components.
The following description more specifically aims, as an example only, at the case of components of Schottky diode type made in vertical form in silicon substrates. However, the present invention also applies to any vertical unipolar structure and to its monolithic forming in a semiconductor substrate, and especially to the forming of MOS-type vertical transistors having a smaller on-state resistance and a greater reverse breakdown voltage in the off state.
2. Discussion of the Related Art
Conventionally, a Schottky diode comprises a heavily-doped semiconductor substrate, typically made of single-crystal silicon. A cathode layer more lightly doped than the substrate covers the substrate. A metal layer or more currently a metal silicide forms a Schottky contact with the cathode and forms the diode anode.
The forming of such unipolar components comes up against two opposite constraints. Said components must exhibit the lowest possible on-state resistance (Ron) while having a high breakdown voltage. Minimizing the on-state resistance requires minimizing the thickness of the less doped layer and maximizing the doping of this layer. Conversely, to obtain a high reverse breakdown voltage, the doping of the less doped layer should be minimized and its thickness should be maximized, while avoiding creation of areas in which the equipotential surfaces are strongly bent.
Various solutions have been provided to reconcile these opposite constraints, which has led to the obtaining of MOS-capacitance Schottky diode structures, currently designated as TMBS, for Trench MOS Barrier Schottky. In an example of such structures, conductive areas, for example, heavily-doped N-type polysilicon areas, are formed in an upper portion of a thick cathode layer less heavily N-type doped than an underlying substrate. An insulating layer insulates the conductive areas from the thick layer. An anode layer covers the entire structure, contacting the upper surface of the insulated conductive areas and forming a Schottky contact with the cathode.
In reverse biasing, the insulated conductive areas cause a lateral depletion of the cathode layer, which modifies the distribution of the equipotential surfaces in this layer. This increases the cathode layer doping, and thus reducing the on-state resistance with no adverse effect on the reverse breakdown voltage.
FIGS. 1 and 2 are partial views of examples of TMBS Schottky diodes of prior art. In these two drawings, a portion of the component has been shown on the right-hand side and the component periphery intended to ensure its voltage capacity has been shown on the right-hand side.
In these two drawings, the component shown on the right-hand side is formed from a heavily-doped N-type silicon wafer 1 on which is formed a lightly-doped N-type epitaxial layer 2. In this epitaxial layer, in the area corresponding to the actual component, are formed trenches having their walls coated with an oxide layer 3 and which are filled with polysilicon 4 doped to be conductive. Conventionally, the oxidation may be a thermal oxidation and the polysilicon filling may be performed by conformal deposition, these filling steps being followed by a planarization step. After this, a metal, for example, nickel capable of forming a silicide 5 above the single-crystal silicon regions and 6 above the polysilicon filling areas, is deposited. Once the silicide has been formed, the metal which has not reacted with the silicon is removed by selective etch. After this, a metal anode deposition 7 is formed.
FIG. 1 illustrates a first example of a peripheral structure enabling spreading of the field lines and the voltage hold of the device. In this example, at the same time as the trenches of the active area, a wider trench 10 is dug at the periphery. The walls of trench 10 are coated with an oxide 13 and with polysilicon 14 formed at the same time as oxide 3 and polysilicon 4. If the polysilicon is conformally deposited and the excess polysilicon is eliminated by anisotropic etch, there only remains polysilicon 14 on the sides of trench 10. The forming of the periphery thus requires no additional step with respect to the forming of the active diode portion. On forming of the silicide, silicide areas also form on external periphery 15 of the component and on polysilicon walls 14 on the trench sides to provide a silicide 16. Then, anode metal layer 7 is deposited and etched so that it comprises an extension which substantially stops in the middle of wide trench 10.
This method for forming the structure of FIG. 1, though apparently simple, however has two disadvantages. On the one hand, it is not easy to etch metal layer 7, for example, aluminum, at the bottom of trench 10. On the other hand, in a practical case where the trench has a depth on the order of from 3 to 10 μm and metal layer 7 has a thickness on the order of from 5 to 10 μm, the metal may poorly deposit on sides 17 of the trench and split. The metal continuity between the upper portion and the bottom of the trench is no longer ensured and the device is not operational.
Another example of a peripheral protection structure according to prior art is shown in FIG. 2. This time, the structure periphery comprises an insulator layer 23, formed, for example at the same time as silicon oxide 2 of the trench walls. Metallization 7 comprises an extension 27 which partially covers this insulation layer. This structure is operative but requires an additional mask level to delimit insulation layer 23.